An effective and reliable collision avoidance system which can be produced and utilized at relatively low cost would be helpful in preventing vehicular accidents either in an open environment, such as on the open highway, or in a closed environment, such as in a mine, or the like. It is desirable that techniques be devised which avoid unwanted interference with undesired electronic or other signals involved, while at the same time giving clear and accurate indications of the distance (range) between two objects, at least one of which is moving, e.g., motor vehicles, and the closing (or opening) rate between the vehicles (i.e., the rate of change of such distance) so that, when the closing rate exceeds selected threshold levels at specified ranges, appropriate action can be taken to avoid collision therebetween. For example, such a system can provide an audible or a visual alarm, or both, to alert the driver of one of the vehicles to the danger of the situation, or the system can be used to take automatic action, e.g., to actuate an automatic vehicular braking system, in order to avoid collision.